What Color Is Your Roof? White Wins Out

According to the Lawrence Berkeley National Laboratory, a simple white roof proves the most cost-effective over a 50-year time span because it reflects three times as much sunshine as a green rooftop garden.

The new report—"Economic Comparison of White, Green, and Black Flat Roofs in the United States"—analyzes 22 commercial, flat-roof projects in the United States in which two or more roof types were considered. The researchers conducted a 50-year life cycle cost analysis, assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs. The study will appear in the March 2014 volume of Energy and Buildings and has just been published online.

"White roofs win based on the purely economic factors we included, and black roofs should be phased out," said study co-author Arthur Rosenfeld, a Berkeley Lab Distinguished Scientist Emeritus and former Commissioner of the California Energy Commission. "Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs."

By absorbing less sunlight than either green or black roofs, white roofs offset a portion of the warming effect from greenhouse gas emissions. "By absorbing less sunlight than either green or black roofs, white roofs offset a portion of the warming effect from greenhouse gas emissions," the lab said.

Green, vegetated roofs, however, provide stormwater management—an appreciable benefit in cities with sewage overflow issues—and help cool the roof's surface as well as the air. Green roofs may also give building occupants the opportunity to enjoy green space where they live or work. While the high installation cost and ongoing maintenance of green roofs set them back in economic terms, their environmental and amenity benefits may at least partially mitigate their financial burden.

The study further noted that plain black roofs can be dangerous because they absorb heat. "White doesn't win out over black by that much in economic terms, so government has a role to ban or phase out the use of black or dark roofs, at least in warm climates, because they pose a large negative health risk," Rosenfeld said.

The study analyzes 22 commercial flat roof projects in the United States in which two or more roof types were considered. The researchers conducted a 50-year life cycle cost analysis, assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs. "We leave open the possibility that other factors may make green roofs more attractive or more beneficial options in certain scenarios," said Mandel, a graduate student researcher at Berkeley Lab. "The relative costs and benefits do vary by circumstance."

White roofs are most cost-effective

The costs and benefits difference stack with the highest net present value shows the most cost-effective type of roof. Parentheses around dollar values indicate negative values. The 50-year life-cycle cost analysis found that even the most inexpensive kind of green roof (with no public access and consisting of only sedum, or prairie grass) costs $7 per square foot more than a black roof over 50 years, while a white roof saves $2 per square foot compared to a black one. In other words, white roofs cost $9 per square foot less than green roofs over 50 years, or $0.30 per square foot each year.